enb,mac_nr: fix MAC PDU packing by tyring to pack multiple SDUs when possible

the patch adds support to fully use the available space in a MAC PDU by adding
multiple SDUs.

srsenb/hdr/stack/mac/nr/ue_nr.h

@@ -96,6 +96,9 @@ private:
                                ue_rx_pdu_queue; ///< currently only DCH PDUs supported (add BCH, PCH, etc)
   srsran::unique_byte_buffer_t ue_rlc_buffer;
 
+  static constexpr int32_t MIN_RLC_PDU_LEN =
+      5; ///< minimum bytes that need to be available in a MAC PDU for attempting to add another RLC SDU
+
   // Mutexes
   std::mutex mutex;
 };

srsenb/src/stack/mac/nr/ue_nr.cc

@@ -76,8 +76,7 @@ int ue_nr::process_pdu(srsran::unique_byte_buffer_t pdu)
 
   // Reverse the order in which MAC subPDUs get processed.
   // First, process MAC CEs, then MAC MAC subPDUs with MAC SDUs
-  for (uint32_t n = mac_pdu_ul.get_num_subpdus(), i = mac_pdu_ul.get_num_subpdus() - 1; n > 0;
-       --n, i = n - 1) {
+  for (uint32_t n = mac_pdu_ul.get_num_subpdus(), i = mac_pdu_ul.get_num_subpdus() - 1; n > 0; --n, i = n - 1) {
     srsran::mac_sch_subpdu_nr subpdu = mac_pdu_ul.get_subpdu(i);
     logger.debug("Handling subPDU %d/%d: lcid=%d, sdu_len=%d",
                  i,
@@ -145,30 +144,58 @@ int ue_nr::generate_pdu(srsran::byte_buffer_t* pdu, uint32_t grant_size)
     return SRSRAN_ERROR;
   }
 
-  // read RLC PDU
-  ue_rlc_buffer->clear();
-  int lcid = 4;
-  int pdu_len =
-      rlc->read_pdu(rnti, lcid, ue_rlc_buffer->msg, grant_size - mac_pdu_dl.size_header_sdu(lcid, grant_size));
+  bool drb_activity = false; // inform RRC about user activity if true
+  int  lcid         = 4;     // only supporting single DRB right now
 
-  // Only create PDU if RLC has something to tx
-  if (pdu_len > 0) {
-    logger.debug("Adding MAC PDU for RNTI=%d", rnti);
-    ue_rlc_buffer->N_bytes = pdu_len;
-    logger.debug(ue_rlc_buffer->msg, ue_rlc_buffer->N_bytes, "Read %d B from RLC", ue_rlc_buffer->N_bytes);
+  int32_t remaining_len = mac_pdu_dl.get_remaing_len();
 
-    // add to MAC PDU and pack
-    mac_pdu_dl.add_sdu(lcid, ue_rlc_buffer->msg, ue_rlc_buffer->N_bytes);
+  logger.debug("Adding MAC PDU for RNTI=%d (max %d B)", rnti, remaining_len);
+  while (remaining_len >= MIN_RLC_PDU_LEN) {
+    // clear read buffer
+    ue_rlc_buffer->clear();
 
-    // Indicate DRB activity in DL to RRC
-    if (lcid > 3) {
-      rrc->set_activity_user(rnti);
-      logger.debug("DL activity rnti=0x%x, n_bytes=%d", rnti, ue_rlc_buffer->N_bytes);
+    // Determine space for RLC
+    remaining_len -= remaining_len >= srsran::mac_sch_subpdu_nr::MAC_SUBHEADER_LEN_THRESHOLD ? 3 : 2;
+
+    // read RLC PDU
+    int pdu_len = rlc->read_pdu(rnti, lcid, ue_rlc_buffer->msg, remaining_len);
+
+    if (pdu_len > remaining_len) {
+      logger.error("Can't add SDU of %d B. Available space %d B", pdu_len, remaining_len);
+      break;
+    } else {
+      // Add SDU if RLC has something to tx
+      if (pdu_len > 0) {
+        ue_rlc_buffer->N_bytes = pdu_len;
+        logger.debug(ue_rlc_buffer->msg, ue_rlc_buffer->N_bytes, "Read %d B from RLC", ue_rlc_buffer->N_bytes);
+
+        // add to MAC PDU and pack
+        if (mac_pdu_dl.add_sdu(lcid, ue_rlc_buffer->msg, ue_rlc_buffer->N_bytes) != SRSRAN_SUCCESS) {
+          logger.error("Error packing MAC PDU");
+          break;
+        }
+
+        // set DRB activity flag but only notify RRC once
+        if (lcid > 3) {
+          drb_activity = true;
+        }
+      } else {
+        break;
+      }
+
+      remaining_len -= pdu_len;
+      logger.debug("%d B remaining PDU", remaining_len);
     }
   }
 
   mac_pdu_dl.pack();
 
+  if (drb_activity) {
+    // Indicate DRB activity in DL to RRC
+    rrc->set_activity_user(rnti);
+    logger.debug("DL activity rnti=0x%x", rnti);
+  }
+
   if (logger.info.enabled()) {
     fmt::memory_buffer str_buffer;
     mac_pdu_dl.to_string(str_buffer);